Add indentation of "return" in sequence diagrams's activated blocks

The return keyword takes an optional label. Use it as shorthand for
"deactivate X[: LABEL]" when the block you want to deactivate is the
most recent activated one.

plantuml-mode.el

@@ -610,7 +610,7 @@ or it is followed by line end.")
       (defvar plantuml-indent-regexp-block-end "^\s*\\(?:}\\|endif\\|else\s*.*\\|end\\)\s*\\('.*\\)?$")
       (defvar plantuml-indent-regexp-note-end "^\s*\\(end\s+note\\|end[rh]note\\)\s*\\('.*\\)?$")
       (defvar plantuml-indent-regexp-group-end "^\s*end\s*\\('.*\\)?$")
-      (defvar plantuml-indent-regexp-activate-end "^\s*deactivate\s+.+$")
+      (defvar plantuml-indent-regexp-activate-end "^\s*\\(?:deactivate\s+.+\\|return.*\\)$")
       (defvar plantuml-indent-regexp-box-end "^\s*end\s+box\s*\\('.*\\)?$")
       (defvar plantuml-indent-regexp-ref-end "^\s*end\s+ref\s*\\('.*\\)?$")
       (defvar plantuml-indent-regexp-title-end "^\s*end\s+title\s*\\('.*\\)?$")

test/plantuml-indentation-sequence-test.el

@@ -302,6 +302,38 @@ activate participant_1
 deactivate participant_1
 "))
 
+(ert-deftest plantuml-test-block-indentation/sequence/activate-return ()
+  "Test correct indentation of an activate-return block."
+  (plantuml-test-indent-block
+   "
+activate participant_1
+participant_1 -> participant_2 : f()
+return
+"
+   "
+activate participant_1
+  participant_1 -> participant_2 : f()
+return
+"))
+
+(ert-deftest plantuml-test-block-indentation/sequence/activate-return-nested ()
+  "Test correct indentation of two nested activate-deactivate blocks."
+  (plantuml-test-indent-block
+   "
+activate participant_1
+activate participant_2
+participant_1 -> participant_2 : f()
+return with a label
+return
+"
+   "
+activate participant_1
+  activate participant_2
+    participant_1 -> participant_2 : f()
+  return with a label
+return
+"))
+
 
 (ert-deftest plantuml-test-indentation/sequence-diagram ()
   "Test correct indentation of plantuml sequence diagram elements.
@@ -356,6 +388,60 @@ else Another type of failure
 end
 "))
 
+(ert-deftest plantuml-test-indentation/nested-sequence-diagram ()
+  "Test correct indentation of plantuml nested sequence diagram elements."
+  (plantuml-test-indent-block
+   "
+A -> foo
+activate foo
+	foo --> bar
+	activate bar
+        bar->baz
+	activate baz
+	alt success case
+        baz --> baz: success
+else
+        baz --> baz: fail
+end
+return
+bar->quux
+activate quux
+        note over quux
+        Cum sociis natoque
+        penatibus et magnis
+        dis parturient montes,
+        nascetur ridiculus mus.
+end note
+return
+return
+return
+"
+   "
+A -> foo
+activate foo
+  foo --> bar
+  activate bar
+    bar->baz
+    activate baz
+      alt success case
+        baz --> baz: success
+      else
+        baz --> baz: fail
+      end
+    return
+    bar->quux
+    activate quux
+      note over quux
+        Cum sociis natoque
+        penatibus et magnis
+        dis parturient montes,
+        nascetur ridiculus mus.
+      end note
+    return
+  return
+return
+"))
+
 
 (provide 'plantuml-indentation-sequence-test)